Active control precision damping table

ABSTRACT

An active control precision damping table supported by air springs. Level displacement and vibrations of the table are detected by a sensor or sensors, and sensor outputs are fed to a variation adder which carries out an adding operation on one of the sensor outputs and an inversion signal produced by inverting the other sensor output 180 degrees. The adder outputs results of the operation to a drive circuit, which in turn outputs a drive signal to a control valve. The control valve is operable in response to the drive signal to adjust air pressure in pressure vessels of the air springs.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to (1) a displacement detecting circuitfor controlling the posture of a running vehicle or for immediately andaccurately detecting displacement in the level of a precision surfaceplate which supports a holography set, an electron microscope, asemiconductor manufacturing device or any other precision measuringdevice, (2) a novel control circuit with the above detecting circuitincorporated into a variation control circuit which maintains orimproves the precision of devices by preventing or suppressingmicrovibrations transmitted from the floor, (3) a variation controlcircuit including the above control circuit and having a displacementdetecting function, and (4) an active control precision damping tableincluding the above circuit, which table supports a holography set, anelectron microscope, a semiconductor manufacturing device or otherprecision measuring device.

(2) Description of the Prior Art

(1) A known example of a displacement detecting circuit utilizing anencoder will be described. In this example, the encoder has a referencescale attached to a reference surface and a movable scale attached to amovable side. The encoder outputs a pulse signal corresponding to anamount of movement of the movable scale from zero point on the referencescale. The amount of movement is detected through a count of the numberof pulses. The precision and resolution in detecting the amount ofmovement are limited since they are dependent on the mechanicalprecision and mechanical construction of the pulse motor. To improve thedetecting precision, the encoder per se must be increased in size. Therehas also been the problem of detection of movement being limited by theencoder capacity.

(2) A known variation control circuit simply detects a disturbancetransmitted from the floor to a measured object, and outputs a signalcorresponding to the disturbance to a drive circuit for controlling thedisturbance. There has been no measure taken to deal with levelvariations due to load displacement or a shift in the center of gravityon the vehicle or the surface plate.

(3) Controls of the displacement and shift require a level sensor and avariation sensor, which would complicate the circuit construction.

(4) Known damping tables have all relied simply on a damping systememploying support by air springs. This system produces a conspicuousdamping effect in a high frequency region but allows resonance in a lowfrequency region. When an impact occurs, it is not immediately absorbedbut produces a very large amplitude as shown in FIG. 11. There is aproblem of slow damping speed with the shock gradually attenuatingthereafter to a certain level. There have been proposals to effectactive control by means of a control valve or the like, but theseproposals are hardly practicable since they fail to deal with levelvariations due to load variation or shift.

SUMMARY OF THE INVENTION

The present invention has been made having regard to the disadvantagesof the prior art. An object of the present invention is to provide adisplacement detecting circuit capable of electrically counting alldisplacements of an object to realize a high degree of detectingprecision and resolution by use of a small apparatus. Furthermore thereis provided a variation control circuit which utilizes the above circuitto control disturbances and to deal with load variations and shifts inthe center of gravity.

Another object of the present invention is to provide a variationcontrol circuit having a displacement detecting function, which iscapable of electrically counting all displacements of an object torealize a high degree of detecting precision and resolution to be verycompact. Such a circuit is capable of controlling disturbances inaddition to detecting displacements by means of a single sensor, and asmall simple circuit structure.

Still another object of the invention is to provide an active controlprecision damping table including the above circuit, which, althoughutilizing known air springs, is capable of precise level control andvibration damping control in a manner basically different from the priorart.

In order to achieve the objects, a displacement detecting circuitaccording to the present invention comprises a displacement sensingsection for detecting displacement of a device and outputting adisplacement signal voltage, a displacement pulse generator forgenerating constant pulses in accordance with an internal clock whilethe displacement signal voltage is output, and an adder for adding thepulses thereby to indicate the displacement as a digital value.

In the above construction, the displacement sensing section firstdetects displacement such as the level of a vehicle, device or othermeasured object, and outputs an analog displacement signal voltagecorresponding to the displacement. This analog displacement signalvoltage is input to the displacement pulse generator. The pulsegenerator generates constant pulses in accordance with an internal clockwhile the displacement signal voltage is output. These pulses are inputto the adder which counts the pulses and outputs a displacement signal.This forms a basis for determining the displacement as a digital value.

Thus, in the displacement detecting circuit according to the presentinvention, the displacement detecting precision and resolution areelectrically determined as distinct from the known encoder system. Thisdetecting circuit is highly responsive as well as assuring highprecision. There occurs no scale overage regardless of the amount ofdisplacement, and there is a further advantage of compactness incapacity. Scale overage is a term used when an instrument is movedbeyond a point at which a displacement cannot any longer be detected.

In order to achieve a portion of the objects, a variation controlcircuit utilizing the above displacement detecting circuit according tothe present invention comprises a vibration detecting circuit fordetecting vibrations of a measured object due to a vibration source suchas a floor, a machine or the like, an arithmetic circuit for outputtinga variation detection signal in response to a variation signal receivedfrom the vibration detecting circuit, a phase inverter for invertingphase of the variation detecting signal 180 degrees, a level variationadder for adding an inversion signal output from the phase inverter anda level variation signal output from the adder of the displacementdetecting circuit, and a drive circuit for generating a drive signal inresponse to an added variation signal received from the level variationadder.

In the above construction, the vibration detecting circuit detectsvibrations of a measured object due to a vibration source such as afloor or a device. In response to a variation signal received from thevibration detecting circuit, the arithmetic circuit outputs an analogvariation detection signal. The phase of this detection signal isinverted 180 degrees by the phase inverter. The level variation adderadds an inversion signal output from the phase inverter and the leveldisplacement signal output from the adder of the displacement detectingcircuit noted hereinbefore, to effect level compensation as well asdisturbance control. An added variation signal as compensated is inputto the drive circuit which outputs a level-compensated disturbancecontrol drive signal to a drive mechanism.

Thus, in the variation control circuit according to the presentinvention, the phase inverter allows not only the vibrations from thevibration source to be offset but the return in level to be madepromptly. This control circuit is very well suited for practical use.

In order to achieve other objects, a variation control circuit having adisplacement detecting function according to the present inventioncomprises a displacement detecting circuit including a sensor fordetecting level displacement of a device and vibrations of the devicedue to a vibration source and outputting a displacement or variationsignal voltage, a comparator for comparing the displacement signalvoltage with a reference voltage and outputting a displacement signalvoltage when the displacement signal voltage deviates from the referencevoltage, a displacement pulse generator for generating constant pulsesin accordance with an internal clock while the displacement signalvoltage is output, and an adder for adding the pulses thereby toindicate the displacement as a digital value; an arithmetic circuit foroutputting a variation detection signal in response to a variationsignal received from the sensor as divided; a phase inverter forinverting the phase of the variation detecton signal 180 degrees; alevel variation adder for adding an inversion signal output from thephase inverter and a level variation signal output from the adder of thedisplacement detecting circuit; and a drive circuit for generating adrive signal in response to an added variation signal received from thelevel variation adder.

The circuit for achieving an object includes two sensors, namely thelevel sensor and vibration sensor, whereas the circuit for achieving theanother object includes a single sensor acting as both the level sensorand vibration sensor.

On the other hand, the displacement detecting circuit is the same inboth constructions. Level displacement of a device detected by thesensor is output as a displacement signal after the pulses are countedby the adder as described hereinbefore, whereby by the displacement isgrasped as a digital value.

This sensor also detects vibrations from the floor, device or othervibration source. The resulting amount of variation is divided as avariation signal for an input to the arithmetic circuit. In response tothe variation signal, the arithmetic circuit outputs an analog variationdetection signal. The subsequent process is the same as in the circuitfor achieving the first object, to effect disturbance control and levelcompensation.

The level displacement and the variation due to the vibration source asdetected by the sensor may be divided for input to the comparator andthe arithmetic circuit and output therefrom simultaneously to be addedby the level variation adder as noted above. However, a controller maybe provided to actuate the comparator for level compensation andthereafter to actuate the arithmetic circuit for disturbance control.

Thus, the variation control circuit has an advantage, in addition to theadvantage of the control circuit, of requiring a single sensor to effectlevel adjustment and disturbance vibration damping. This feature allowsthe circuit construction to be very simple.

An embodiment of an active control precision damper table according tothe present invention comprises a table body supported by air springshaving pressure vessels. On the table body, there are provided a levelsensor and a vibration sensor. A level displacement signal is detectedby the level sensor and an inversion signal is obtained by converting avibration detecting signal resulting from vibrations of the table bodycaused by a vibration source such as a floor 180 degrees. The leveldisplacement signal and the inversion signal are applied to a levelvariation adder where these signals are added together. A drive circuitgenerates a drive signal and controls opening and closing of a controlvalve which adjusts air pressure in the pressure vessels of the airsprings.

An embodiment of an active control precision damper table according tothe present invention comprises a table body supported by air springshaving pressure vessels. On the table body there are included a sensor,a level variation adder for carrying out an adding and subtractingoperation on an inversion signal produced by inverting a vibrationdetection signal output from the sensor 180 degrees resulting fromvibrations of the table body due to a vibration source such as a floor,a device or the like. A drive circuit generates a drive signal inresponse to an added variation signal received from the level variationadder, and controls opening and closing of a control valve operable inresponse to the drive signal to adjust air pressure in the pressurevessels of the air springs.

In the first type of active control precision damper table, when a loadsuch as a device is placed on the table body, the latter moves downwardby an amount corresponding to the load. Then the level sensorimmediately detects the level displacement δ, and outputs a leveldisplacement signal corresponding to the displacement. This leveldisplacement signal is processed and input to the level variation adder.At the same time, the vibration sensor detects vibrations of the tablebody due to a vibration source such as the floor any other device. Theresulting detection signal is inverted 180 degrees into an inversionsignal for input to the level variation adder. The level variation addercarries out an adding operation on the inversion signal and the leveldisplacement signal, and outputs an added variation signal with levelcompensation as well as disturbance control. The added variation signalas compensated is input to the drive circuit. The drive circuit outputsa level-compensated disturbance control drive signal for precisioncontrol of an opening and closing degree of the control valve.

These processes are the same as in the variation control circuit thatachieves the second half of the first object of the invention.Therefore, the phase inverter immediately cancels out the vibrationsfrom the vibration source, to produce a very precise and rapid vibrationdamping effect and to effect level compensation for a prompt return tothe original level at the same time. This active control precisiondamper table is quite novel and finds no parallel in the prior art.

The second type of active control precision damper table employs thecircuit that achieves the second object of the invention. As distinctfrom the first type of damper table, the second type damper tableincludes a single sensor in replacement for the level sensor andvibration sensor. The function and effect are the same as in the circuitachieving the second object.

Other advantages of the present invention will be apparent from thefollowing description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a circuit constructed to achieve the first objectof the present invention,

FIG. 2 is a diagram of a precision surface plate employing the circuitshown in FIG. 1,

FIG. 3 is a diagram of a circuit constructed to achieve the secondobject of the invention,

FIG. 4 is a diagram of a precision surface plate employing the circuitshown in FIG. 3,

FIG. 5 is a view illustrating a relationship between variations in thecurrent flowing through a control valve of the present invention and awaveform occuring at control times,

FIG. 6 is a schematic perspective view showing how sensors are arrangedon a damping table,

FIG. 7 is a schematic sectional view of the control valve employed forthe damping table,

FIG. 8 is a graph for comparing damping effects of the damping table ofthe present invention and a known damping table supported by airsprings,

FIG. 9 is a graph showing a standstill state of the damping tablevibration-damped according to the present invention when a shock isapplied thereto,

FIG. 10 is a graph showing a damping effect produced against a shock bya known active damping system having no level compensating fuction, and

FIG. 11 is a graph showing a damping effect produced by known airsprings when an impact load is applied.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention will be described hereinafter first as embodiedinto a displacement detecting circuit A, next a variation controlcircuit B1 which achieves the first object of the present invention,then a variation control circuit B2 which achieves the second object ofthe invention, and finally two types of damping table C1 and C2utilizing these control circuits.

A. One Example of Displacement Detecting Circuit A Shown in FIGS. 1 and2

The displacement detecting circuit A comprises a displacement sensingsection 1, a displacement pulse generator 2 and a displacement detectionadder 3. The sensing section 1 includes a displacement amplifier 12, areference voltage setter 13 and a comparator 14. The displacementamplifier 12 is connected to receive a signal from a sensor such as alevel sensor 11 for detecting displacement of a measured object from areference position. The level sensor 11 detects a downward movement of,for example, a damping table C under a load D, and generates a detectionsignal voltage corresponding to the downward movement. Here the amountof displacement is represented by δ as shown in FIG. 2. The detectionsignal voltage is transmitted through the displacement amplifier 12 tobe input in increased amplitude to the comparator 14. This comparator 14is connected also to the reference voltage setter 13, and the amplifieddetection signal voltage is compared with a reference voltage. When thedetection signal voltage is higher or lower than the reference voltage,the comparator 14 outputs a displacement signal voltage to thedisplacement pulse generator 2. The displacement pulse generator 2continues to generate a constant pulse signal in accordance with aninternal clock during receipt of the displacement signal voltage. Thedisplacement pulse signal is input to the adder 3 where the pulses areadded sequentially, so that the displacement is detected as a digitalvalue. The displacement is output as a stepwise digital signal to a D/Aconverter 15 connected to the adder 3. The D/A converter 15 converts thedigital signal into a smooth displacement analog signal, and applies theanalog signal to a level variation adder 8 of a variation controlcircuit B₁ which is not a part of the displacement detecting circuit A.

The above circuit construction is capable of coping with a greatdisplacement exceeding the capacity of level sensor 11. Moreparticularly, the number of pulses is counted as long as thedisplacement detection voltage differs from the reference voltage, thecounting of pulses being stopped only when a movable element returns toa home position and its displacement becomes zero. Thus a return to zeropoint is made accurately even in the event of overscaling of the levelsensor 11.

B. One Example of Variation Control Circuit B₁

The variation control circuit B₁ as shown in FIGS. 1 and 2 comprises avibration detecting circuit 5, an arithmetic circuit 6, a phase inverter7, a level variation adder 8 and a drive circuit 9. The vibrationdetecting circuit 5 includes a lowpass filter 17. The lowpass filter 17is connected to a vibration sensor 16 on damping table C₁. The vibrationsensor is not a part of the vibration detecting circuit 5.

Vibrations transmitted from a vibration source 4 such as the floor, amachine or the like to a measured object are detected and output as avibration signal voltage by the vibration sensor 16. At this stage, highfrequency components are superposed on the vibration signal voltage. Thehigh frequency components are filtered by the lowpass filter 17 asnecessary, and relatively smooth low frequency components only areoutput as the vibration signal voltage. It is, of course, unnecessary tofilter the high frequency components if a control mechanism describedlater is fully capable of coping with the high frequency components. Thevibration signal voltage having been filtered is input as a vibrationsignal to the arithmetic circuit 6, and is output therefrom as avariation detection signal. This variation detection signal is input tothe phase inverter 7, from which an inversion signal emerges with thephase inverted 180 degrees for an input to the level variation adder 8.

The level variation adder 8 receives the inversion signal output fromthe phase inverter 7 and the level displacement signal output from theadder 3 through D/A connector 15. The displacement detecting circuit A,carries out an adding operation on the two signals, and outputs aresulting signal to a drive section. In response to the added variationsignal received from the level variation adder 8, the drive sectionoutputs a drive signal for causing the control mechanism to effectposture control and vibration damping.

The above circuit construction as incorporated into the damping table Cwill be described in detail later.

Next, a variation control circuit B₂, as shown in FIGS. 3 and 4 forachieving the second object of the invention will be described. The samecomponents as in the variation control circuit B₁ achieving the firstobject will not be described and only different components will bedescribed.

The displacement detecting circuit A is the same as in the variationcontrol circuit B₁ as already described.

The difference lies in that a single sensor 100 is provided in place ofthe level sensor 11 and the vibration sensor 16. The sensor 100 detectsdisturbances from the floor, vibrations from a load D placed on thedamping table C₂, and level displacements due to these disturbances orvibrations. Resulting signals output from the sensor 100 are amplifiedby the displacement amplifier 12, and are then divided for input to thecomparator 14 of the displacement detecting circuit A and to thearithmetic circuit 6 of the variation control circuit B₂, respectively.A controller 101 may be provided to selectively actuate the comparator14 and the arithmetic circuit 6. In this case the controller 101 isoperable to actuate the comparator 14 for inputting the signal for levelcompensation to the level variation adder 8 as described hereinbefore.Upon completion of this step, the controller 101 is switched to actuatethe arithmetic circuit 6 for performing a vibration damping function.The comparator 14 and the arithmetic circuit 6 may, of course, beactuated simultaneously without using controller 101, the outputs of thecomparator 14 and arithmetic circuit 6 being simultaneously input to thelevel variation adder 8 to carry out the level compensation andvibration damping at the same time.

The level compensation process and damping process will not be describedsince they are the same as in the circuit achieving the first object

Active precision damping tables C₁ and C₂ employing the above twocircuits will be described next.

First, the damping table C₁ which employs the circuit realizing thefirst object will be described with reference to FIGS. 1 and 2, but thedescription of the circuit per se will not be repeated.

The drive circuit 9 receives the level compensation signal and controlsignal output from the level variation adder 8 of the variation controlcircuit B₁. In response to the added variation signal, the drive circuit9 outputs a drive signal to effect precision control of an opening andclosing degree of a control valve 18 comprising a servo valve or aproportional control valve. In this case the control valve 18 comprisesa servo valve. As shown in FIG. 7, a vibration plate 18a is controlledby an electromagnetic coil 18b to precisely control air pressure inpressure vessels 20 of air springs 21. The damping table C1 is supportedby the air springs 21 including the pressure vessels 20, and pressurecontrol of the air springs 21 is effected by the control valve 18 asnoted above.

When a load D shown in phantom is applied to the precision damping tableC₁, the damping table C1 moves downwardly by an amount corresponding tothe load D. The downward movement is immediately detected by the levelsensor 11, which is input to the drive circuit 9 through the describedroute. As a result, the control valve 18 is actuated to supplycompressed air to the pressure vessels 20 thereby to expand the airsprings 21. This drives the damping table C₁ back to a home position.When the damping table C₁ completely returns to the home position, thedisplacement becomes zero whereby the level sensor 11 stops generatingthe displacement signal voltage. Since the number of pulses produced bythe load D is counted by the adder 3 at this time, an energizing currentcorresponding to the load D continues to flow from the drive circuit 9to the control valve 18 (that is, an energizing voltage is applied tothe control valve 18). Consequently, the compressed air is continuouslysupplied to the pressure vessels 20 in an amount to keep lifting theload D to the home position.

When the damping table C₁ is lifted beyond the home position, the levelsensor 11 generates the displacement signal voltage as above. Then thedamping table C₁ is lowered to the home position.

In the variation control circuit B₁, on the other hand, the phaseinverter 7 outputs the inversion signal with the phase inverted 180degrees. Therefore, not only low frequency vibrations generated from thevibration source 4 are canceled out to prevent resonance but shocks, ifany, are rapidly damped. As a result, the damping table C₁ is stoppedsubstantially upon arrival at the original level. This state isillustrated in FIG. 9. FIG. 10 shows a comparative example in whichactive control is effected without the level compensating function. FIG.11 shows the prior art vibration damping by means of air springs. FIG. 8shows the difference between the present invention and the prior art invibration damping by means of air springs.

The embodiment shown in FIG. 4 is slightly different from the embodimentof FIG. 2, and employs the circuit achieving the second object of theinvention. In this embodiment, the level sensor 11 and vibration sensor5 are integrated into the sensor 100, and the lowpass filter 17 isomitted. The output of sensor 100 is divided for input to the arithmeticcircuit 6. The subsequent process is the same as in the foregoingdamping table C₁.

Referring to FIG. 6, where both the level sensor 11 and vibration sensor5 are used, these sensors are provided at a total of six to eightpositions over the entire damping table C₁, with the level sensor 11provided at three or four positions opposed to the legs, and thevibration sensor 5 also at three or four positions. Where the sensor 100is used, it is provided at three or four positions of the damping tableC₁.

The vibration sensors 5 may be provided not only on a top surface but onside faces of the damping table C₁.

The reference character 19 in FIGS. 1 and 3 denotes a regulator whichregulates the compressed air supplied by a compressor to the controlvalve.

What is claimed is:
 1. An active control precision damping tablecomprising a table body supported by air springs having pressurevessels, the table body being provided with a level sensor and avibration sensor, a level variation adder for carrying out an addingoperation on a level displacement signal output received from the levelsensor and a vibration detection signal output from the vibration sensorresulting from vibrations of the table body due to a vibration source inwhich the vibration detection signal is inverted by 180 degrees, a drivecircuit for generating a drive signal in response to an added variationsignal received from the level variation adder, and a control valveoperable in response to the drive signal to adjust air pressure in thepressure vessels of the air springs.
 2. An active control precisiondamping table comprising a table body supported by air springs havingpressure vessels, the table body being provided with a sensor, a levelvariation adder for carrying out an adding operation on a leveldisplacement signal output received from the sensor and a vibrationdetection signal output received from the sensor resulting fromvibrations of the table body due to a vibration in which the vibrationdetection signal is inverted by 180 degrees, a drive circuit forgenerating a drive signal in response to an added variation signalreceived from the level variation adder, and a control valve operable inresponse to the drive signal to adjust air pressure in the pressurevessels of the air springs.